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BIG PHYSICS, BIG QUESTIONS –

The oceans are heating, acidifying and choking

By Fred Pearce

WE KNOW the oceans are warming. And we know they are acidifying. Now, to cap it all, it turns out they are suffocating, too. A new health check on the state of the oceans warns that the oceans will have lost as much as 7 per cent of their oxygen content by the end of the century.

“The health of the oceans is spiralling downwards far more rapidly than we had thought,” says Alex Rogers at the University of Oxford, the scientific director of IPSO. “Things are happening that are unprecedented, and exposing organisms to intolerable and unpredictable evolutionary pressure,” he says.

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The health of the oceans is spiralling downwards far more rapidly than we had thought

Rogers describes a “deadly trio” of linked global threats. The first is global warming&colon; surface seawater has been warming almost as fast as the atmosphere. The second is acidification, which is a result of the oceans absorbing ever more carbon dioxide from the atmosphere. The third is deoxygenation.

The oceans are losing oxygen partly because warmer water holds less gas, and partly because warming is greatest at the surface. This creates a buoyant surface layer that mixes less with colder layers below, resulting in oxygen-poor deep water that can suffocate the seabed, say the authors.

Near coasts, deoxygenated water from the depths can still return to the surface, though, carried by upwelling currents, says Rogers. This could kill marine life in shallow water too. In fact, the report blames oxygen-poor water brought to the surface by the California Current for a massive loss of marine life off North America in the past decade. “This region showed no evidence of hypoxia [low oxygen levels] prior to 2000,” it says.

Meanwhile, tagging studies have shown that large fish like marlin, which need a lot of oxygen to fuel their fast metabolisms, are disappearing from some of the areas in the tropical oceans where oxygen is diminishing.

Ralph Keeling of the Scripps Institution of Oceanography in La Jolla, California, predicts that the oceans will lose between 1 and 7 per cent of their oxygen this century.

“We know that low oxygen levels have occurred in previous eras when there were mass extinctions, and now we are seeing signs of a repeat,” says Rogers.

Other oceanographers say that the report’s claims of rapid recent deterioration are exaggerated. “To say things are dramatically worse than two years ago is hype,” says Callum Roberts of the University of York, UK. But he agrees that the threat from the deadly trio is real. “The ocean deserts are expanding. We have a squeeze on ocean productivity that will reduce fisheries’ potential in the coming decades if we don’t reduce carbon dioxide emissions,” he says.

This article appeared in print under the headline “Choking oceans complete ‘deadly trio’ for marine life”